Williams 
Still another contemporary
of D'Alembert and Delambre, and somewhat older than either of them, was
Leonard Euler (17071783), of Basel, whose fame as a philosopher equals
that of either of the great Frenchmen. He is of particular interest here
in his capacity of astronomer, but astronomy was only one of the many fields
of science in which he shone. Surely something out of the ordinary was
to be expected of the man who could "repeat the AEneid of Virgil from the
beginning to the end without hesitation, and indicate the first and last
line of every page of the edition which he used." Something was expected,
and he fulfilled these expectations.
In early life he devoted himself to the
study of theology and the Oriental languages, at the request of his father,
but his love of mathematics proved too strong, and, with his father's consent,
he finally gave up his classical studies and turned to his favorite study,
geometry. In 1727 he was invited by Catharine I. to reside in St. Petersburg,
and on accepting this invitation he was made an associate of the Academy
of Sciences. A little later he was made professor of physics, and in 1733
professor of mathematics. In 1735 he solved a problem in three days which
some of the eminent mathematicians would not undertake under several months.
In 1741 Frederick the Great invited him to Berlin, where he soon became
a member of the Academy of Sciences and professor of mathematics; but in
1766 he returned to St. Petersburg. Towards the close of his life be became
virtually blind, being obliged to dictate his thoughts, sometimes to persons
entirely ignorant of the subject in hand. Nevertheless, his remarkable
memory, still further heightened by his blindness, enabled him to carry
out the elaborate computations frequently involved.
Euler's first memoir, transmitted to the
Academy of Sciences of Paris in 1747, was on the planetary perturbations.
This memoir carried off the prize that had been offered for the analytical
theory of the motions of Jupiter and Saturn. Other memoirs followed, one
in 1749 and another in 1750, with further expansions of the same subject.
As some slight errors were found in these, such as a mistake in some of
the formulae expressing the secular and periodic inequalities, the academy
proposed the same subject for the prize of 1752. Euler again competed,
and won this prize also. The contents of this memoir laid the foundation
for the subsequent demonstration of the permanent stability of the planetary
system by Laplace and Lagrange.
It was Euler also who demonstrated that
within certain fixed limits the eccentricities and places of the aphelia
of Saturn and Jupiter are subject to constant variation, and he calculated
that after a lapse of about thirty thousand years the elements of the orbits
of these two planets recover their original values.. 
